Community Forums

twinboat · 07-28-2018, 05:06 AM

Quote:

Originally Posted by Soppy

I would use #4 wire for that long of a run . Although #6 would work , You still have the length of the cord on the motorhome which will further add to the voltage drop. An A/C unit will draw at least 4 times its running amps to start , so a 15,000 BTU unit will require about 50+ amps to start . If your AC voltage is a little on the low side , it will drop even more when starting the air conditioner.

Wow, my 15000 BTU AC starts and I only have 30 amp service.

I think the start capacitor is there to assist in that higher amp startup draw.

Butte64 · 07-28-2018, 05:12 AM

For anyone needing 30 A from a 50 A outlet there are several adapters available including one from Amazon for $13.

xrated · 07-28-2018, 05:23 AM

Quote:

Originally Posted by spuds

Reference, please.

All documents I can find for a 15K RV a/c show numbers in the range of 14-18 amps on start up.

Official Reference...........I've been a Journeyman Electrician since 1980, and yes, it will start at the Locked Rotor Current Rating. An example of that is the Brisk II A/C unit in my Toy Hauler....
Full Load current for the compressor, not including the fan....12.4 amps
Locked Rotor Current.....68 amps

Fan Full Load Rating.....2.5 amps
Fan Locked Rotor....5.8 amps

So for a short time......milli-seconds, almost 74 amps

And this is for a 13.5K BTU A/C unit.....the 15K BTU units will be slightly higher.

Sbrownstein · 07-28-2018, 05:35 AM

I am sure that there are transients that go very high. When a compressor starts, in effect the rotor is stationary which is equivalent to "locked." However, millisecond events don't trigger breakers and are not at all involved in wire size calculations. Arc welders are allowed to have undersized cabling based on duty factor and over-sized breakers and that isn't milliseconds it's 25 or 30 minutes per hour. Use the table values...this is not a transient exercise.

Soppy · 07-28-2018, 05:56 AM

Quote:

Originally Posted by fla tom

Not to question to much but most coaches(our last 4 included) have 3 15k ac units ,our coaches only have 100 amp service at most but all three will start and run at the same time(by your estimate that would be minimum 150 amps) .our current motorhome has three 15k btu heat pumps each on a 20 amp breaker .would that not be 60 amps max draw ?

Starting an induction motor (compressor) can require from 3 to 7 times the running current , depending on the motor design. The 150 amps will occur at startup . This high current draw is only for a few miliseconds so a breaker or fuse will not trip. Circuit breakers are designed to handle this startup surge. That is why you will see 120v lights dim or flicker when the A/C units start.

oneknight · 07-28-2018, 05:58 AM

When I did mine, I used alluminum SER cable. "SER" = Service Entrance. Stuff generally used to run from the meter to a trailer or mobile home. Bought it online, cheap as hell. Lowes and home depot generally stock it in our area.

I used 2-2-2-4 to run from the house box through the crawlspace to a new 100 amp subpanel in the garage, then 4-4-4-6 up, over, and down across the garage to a 50 amp box from lowes.

I like this alluminum SER cable a lot. It's light, easy to bend, dooesn't need to be in any kind of conduit. It's not rated for underground though. And yes, I check the tightness of all the terminal periodically.

Props2 · 07-28-2018, 06:13 PM

Quote:

Originally Posted by oneknight

When I did mine, I used alluminum SER cable. "SER" = Service Entrance. Stuff generally used to run from the meter to a trailer or mobile home. Bought it online, cheap as hell. Lowes and home depot generally stock it in our area.

I used 2-2-2-4 to run from the house box through the crawlspace to a new 100 amp subpanel in the garage, then 4-4-4-6 up, over, and down across the garage to a 50 amp box from lowes.

I like this alluminum SER cable a lot. It's light, easy to bend, dooesn't need to be in any kind of conduit. It's not rated for underground though. And yes, I check the tightness of all the terminal periodically.

Hmmm, First of all, check local codes,most will not allow aluminum wiring for residential sub panels. If you are using 2-2-2-4, your OCP should be 80a at your main panel. 100A panel would require 1/0 Al to meet the ampacity @60c rating.(My disclaimer)

Depends on the jurisdiction having authority!!